Enabling auto-sync in more than one site built off of same repository throws an error

[olayway]

If a user creates a second site using same GH repository (e.g. both sites publish from different subfolder), and he tries to enable auto-sync in it, the app will show an error, as it's only possible to install one webhook with the same handler url. It should work though, so we need to check for the existence of the webhook and skip double installation, but still toggle the auto-sync on and make sure the handler triggers sync for both sites.